The present invention relates to telecine equipment, in particular, to a method and apparatus for converting an optical signal into an electronic video signal, in which the image is scanned by means of cathode ray tube scanning means.
Telecine equipment is used to convert optical images taken from film into electronic video signals which may be either broadcast directly or recorded on video tape for subsequent transmission. Frequently, conversion is effected in real time, using a `continuous motion telecine` in which the film is moved continuously past scanning means. However, it is often necessary or desirable to mix the video output of the telecine with other video signals, for example, electronic subtitles or other signals which provide special video effects. Consequently, it is becoming more frequently the case that electronic mixing techniques rather than optical methods are used, the result then being re-recorded.
Where electronic mixing techniques are used, stability of the image during film-to-tape transfer is essential since picture movement cannot be tolerated when mixing electronically-generated information. The results of picture movement are often seen on television when films are transmitted with the addition of electronic subtitles. The picture moves erratically behind steady subtitles. Although modern telecine equipment has been improved by the utilisation of continuous motion sprocketless film transport, there often remains some residual movement of the image.
Consequently, when the best possible picture quality is required, it is necessary to transfer the film to video tape in non-real time. This is achieved by fitting a pin-registered gate to the telecine and operating the system with a special video tape recorder which allows non-real time recording.
The pin-registered gate enables film to be held stationary in the gate by pins which are inserted into the same film sprocket holes as were used by the original film camera. The film transport mechanism of the telecine must be modified where a pin-registered gate is used so that the film is fed through the gate frame-by-frame with an intermittent motion rather than with the more usual continuous motion. The pin-registered gate arrangement is not readily utilized in a line-array CCD telecine where the film motion provides vertical scanning of the image. While it would be possible, by the addition of an oscillating mirror placed in the optical path between the film and the line array sensor, to produce a live image from a line-array CCD telecine, the performance would not be as good. The pin-registered gate is, however, ideally suited to the flying spot telecine as it is capable of producing live images from still pictures.
The use of non-real time scanning where electronic mixing techniques are to be used, or for other purposes, also provides an opportunity for scanning each frame several times for the purpose of improving the signal-to-noise ratio of the telecine output.
The main principle underlying noise reduction is that by averaging the signals from successive scans of the same image, the real or desired picture information is reinforced while the noise, due to its random nature, is reduced by the averaging process. FIGS. 1 and 2 of the drawings shows two simple forms of noise reduction filter.
FIG. 1 is a simple transversal filter. The signals resulting from each of five scanning operations are passed to the filter on input line 10. Each of the blocks 12 is a one picture delay. Once the five scanning operations are complete, the signals are combined by an adder 14 and the resultant signal is then divided by five for transmission or recording. The signal-to-noise ratio is greatly improved by this averaging operation. For example, using only two scans for each image gives a 3 dB improvement.
FIG. 2 shows a typical recursive filter 20. In this type of filter, two multipliers, one (XK) in the input path and the other X(1-K) in the output of picture store 22 X(1-K) control the addition proportions of the signal. The output from the picture store 22 is multiplied and added by means of an adder 24 to the incoming signal resulting from the next subsequent scanning operation. If K=1 then there is no noise reduction. If K=0.1, then a very high degree of signal averaging occurs.
Although both the filter arrangements outlined above are effective in reducing noise electronically generated in the telecine processing channel, they do not reduce noise arising in the flying spot cathode ray tube (`CRT`). Such noise arises from phosphor grains and blemishes on the CRT screen. This type of noise is coherent with the picture information and is therefore built up, with the picture information, in the averaging operations described above.